Vine: Fix cache directory property of manager in Python bindings

[tphung3]

Proposed Changes

The cache directory of the TaskVine manager is incorrectly assigned to each workflow's staging directory. The property now correctly points to the global cache of TaskVine which is workflow-agnostic.

Merge Checklist

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Check these off to verify you have completed all steps.

• make test Run local tests prior to pushing.
• make format Format source code to comply with lint policies. Note that some lint errors can only be resolved manually (e.g., Python)
• make lint Run lint on source code prior to pushing.
• Manual Update Update the manual to reflect user-visible changes.
• Type Labels Select a github label for the type: bugfix, enhancement, etc.
• Product Labels Select a github label for the product: TaskVine, Makeflow, etc.
• PR RTM Mark your PR as ready to merge.